The present invention relates generally to a system for restricting access to transmitted programming content, and more particularly, to a system for transmitting an encrypted program together with a program identifier which is used by a set-top terminal, together with stored entitlement information, to derive the decryption key necessary to decrypt the program.
As the number of channels available to television viewers has increased, along with the diversity of the programming content available on such channels, it has become increasingly challenging for service providers, such as cable television operators and digital satellite service operators, to offer packages of channels and programs that satisfy the majority of the television viewing population. The development of packages that may be offered to customers is generally a marketing function. Generally, a service provider desires to offer packages of various sizes, from a single program to all the programs, and various combinations in between.
The service provider typically broadcasts the television programs from a transmitter, often referred to as the xe2x80x9chead-end,xe2x80x9d to a large population of customers. Each customer is typically entitled only to a subset of the received programming, associated with purchased packages. In a wireless broadcast environment, for example, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish. Thus, in order to restrict access to a transmitted program to authorized customers who have purchased the required package, the service provider typically encrypts the transmitted programs and provides the customer with a set-top terminal (STT) containing one or more decryption keys which may be utilized to decrypt programs that a customer is entitled to. In this manner, the set-top terminal receives encrypted transmissions and decrypts the programs that the customer is entitled to, but nothing else.
In order to minimize piracy of the highly sensitive information stored in the set-top terminals, including the stored decryption keys, the set-top terminals typically contain a secure processor and secure memory, typically having a capacity on the order of a few kilobits, to store the decryption keys. The secure memory is generally non-volatile, and tamper-resistant. In addition, the secure memory is preferably writable, so that the keys may be reprogrammed as desired, for example, for each billing period. The limited secure memory capacity of conventional set-top terminals limits the number of keys that may be stored and thereby limits the number of packages which may be offered by a service provider. It is noted that the number of programs typically broadcast by a service provider during a monthly billing period can be on the order of 200,000.
In one variation, conventional set-top terminals contain a bit vector having a bit entry corresponding to each package of programs offered by the service provider. If a particular customer is entitled to a package, the corresponding bit entry in the bit vector stored in the set-top terminal is set to one (xe2x80x9c1xe2x80x9d). Thereafter, all programs transmitted by the service provider are encrypted with a single key. Upon receipt of a given program, the set-top terminal accesses the bit vector to determine if the corresponding bit entry has been set. If the bit entry has been set, the set-top terminal utilizes a single stored decryption key to decrypt the program. While, in theory, flexibility is achieved in the bit vector scheme by providing a bit entry for each package (a package generally consists of one program), the length of the bit vector would be impractical in a system transmitting many programs in a single billing period. In addition, access control in such a system is provided exclusively by the entries in the bit vector and is not cryptographic. Thus, if a customer is able to overwrite the bit vector, and set all bits to one (xe2x80x9c1xe2x80x9d), then the customer obtains access to all programs.
In a further variation, programs are divided into packages, and all programs in a given package are encrypted using the same key. Again, each package typically corresponds to one television channel. The set-top terminal stores a decryption key for each package the customer is entitled to. Thus, if a program is to be included in a plurality of packages, then the program must be retransmitted for each associated package, with each transmission encrypted with the encryption key corresponding to the particular package. Although the access control is cryptographic, the overhead associated with retransmitting a given program a number of times discourages service providers from placing the same program in a number of packages and thereby limits flexibility in designing packages of programs.
While such previous systems for encrypting and transmitting programming content have been relatively successful in restricting access to authorized customers, they do not permit a service provider, such as a television network, to offer many different packages containing various numbers of programs to customers, without exceeding the limited secure memory capacity of the set-top terminal or significantly increasing the overhead. U.S. patent application Ser. No. 08/912,186, filed Aug. 15, 1997 and assigned to the assignee of the present invention, hereinafter referred to as the xe2x80x9cVspace System,xe2x80x9d discloses a cryptographic method and apparatus for restricting access to transmitted programming content.
Each program in the Vspace System is encrypted by the head-end server prior to transmission, using a program key, Kp. Each of the program keys is a linear combination of a defined set of master keys, M. A program identifier identifying the program is transmitted with the encrypted programming content. The customer""s set-top terminal can derive the decryption key from only the received program identifier, p, and previously stored entitlement information. The Vspace System provides a cryptographic access control mechanism, while permitting flexible packages (since the program does not need to be retransmitted for each associated package) without significantly extending the program header (only the program identifier is transmitted with the program).
Generally, encrypted programming content is transmitted by a service provider using a transmitter, or head-end server, to one or more customers. According to one aspect of the invention, a program identifier, p, used to identify the program is transmitted to the customer with the programming content. Each customer has a set-top terminal or another mechanism to restrict access to the transmitted multimedia information using decryption keys. The set-top terminal receives entitlement information from the head-end, corresponding to one or more packages of programs that the customer is entitled to for a given period.
Each program is encrypted by the head-end server prior to transmission, using a program key, Kp, which may be unique to the program. In addition to transmitting the encrypted program, the head-end server transmits the program identifier, p, to the set-top terminal. The set-top terminal uses the received program identifier, p, together with the stored entitlement information, to derive the decryption key necessary to decrypt the program. In this manner, if a customer is entitled to a particular program, the set-top terminal will be able to derive the encrypted program key, Kp, using the stored and received information, and thereafter use the program key, Kp, to decrypt the encrypted program. In various embodiments, the program identifier, p, can be interleaved with the program portion or transmitted on a separate dedicated control channel.
According to one aspect of the invention, each of the k-bit program keys, Kp, used to encrypt transmitted programs is obtained by applying one or more pseudo-random hash functions, H, to a master key, m. In one implementation, a length-doubling hash function, H, is utilized. Thus, the hash function, H, takes a k-bit binary value and produces a binary value having a length of 2k. The output of the hash function, H, can be represented as a pair of k-bit binary values, H0 and H1, where H0 is referred to as the left half of the output of the hash function, and H1 is the right half of the output of the hash function.
In an illustrative implementation, a program key, Kp, is obtained by recursively applying a hash function, H0 or H1, to the master key, m, depending on the corresponding binary value of each bit position of the program identifier, p. Thus, if the program identifier, p, consists of n bits, one of the hash functions, H0 or H1, is applied for each of the n bit positions of the program identifier, p, depending on the corresponding bit value of the program identifier, p. Initially, one of the hash functions, H0 or H1, is applied to the master key, m, depending on the binary value of the most significant bit of the program identifier, p. Thereafter, for each of the remaining (nxe2x88x921) bit positions, one of the hash functions, H0 or H1, is applied to the result of the previous hash operation, depending on the binary value of the corresponding bit. The calculation of the program key, Kp can be represented as follows:
Kp=Hpn( . . . Hp2(Hp1(m)) . . . ).
The hash operation can be represented in terms of an n-level binary tree, T, referred to as the key tree, with the master key, m, placed at the root of the tree. The tree is generated by applying the hash functions H0 and H1 to each node, until the desired number of tree levels (n) have been created. The program keys, Kp, correspond to the leaf nodes at the bottom level of the tree. The binary index (and likewise the program identifiers, p) associated with each program key, Kp, corresponds to the path through the key tree from the root to the desired leaf node. Thus, the index or label of a given node, u, is the concatenation of the labels on the edges on the path from the root to the node u. T(u) denotes the subtree rooted at node u, or the set of program identifiers, p, corresponding to the leaves in the subtree of node u. For an internal node, u, at depth r in the key tree, with a partial program identifier, p, (u1, . . . , ur), the keys of any program in the subtree T(u) can be computed by activating the hash function n-r times.